In a conventional tractor with a swinging drawbar system, it is common practice for most tractor-implement combinations to provide a standard length of drawbar, i.e. the distance in a longitudinal or fore-and-aft direction that the drawbar extends beyond the rear of the tractor. In some cases the drawbar is not used at all and it must be fully retracted to a stored position. In other cases the implement has a power drive and thus must be connected to the power take-off (PTO) shaft of the tractor. In such cases the drawbar is restrained to preclude its swinging from side to side, or laterally, and in effect is rendered fixed. Power lines are utilized to effect the connection of the power train of the implement to the PTO of the tractor. The power line includes a pair of cardan universal joints, commonly called U-joints, to allow the implement, which is pinned to the free end of the drawbar, to pivot relative to the tractor and still transmit power from the tractor PTO to the implement. The common type of U-joint, while permitting drive through an angle, has non-uniform velocity characteristics which can be compensated for, or cancelled by use of a second U-joint, but only if the angles through which the two U-joints are transmitting power are equal. Of course, when the tractor and implement are oriented in a straight line there are no angular relationships created in the drive line. The equal angles are necessary then only during turns, or when traversing uneven ground wherein the tractor and implement shafts are not aligned with each other. In order for the drive line angles to be equal during turns, the point of attachment of the implement to the drawbar, i.e. the center line of the drawbar hitch hole, must be directly under the mid-point of the U-joint assembly when the tractor-implement combination is in a straight line relationship. Because the U-joint assemblies for different power train implements have lengths which vary, it is important that the drawbar length be adjusted so that the hitch pin hole in the free end of the drawbar will be positioned just below the mid-point of each U-joint assembly. If this relationship is not maintained, vibration and accelerated wear in the cross member of the U-joint assemblies and telescoping parts of the drive line will occur when the tractor and implement are angled relative to each other.
In the prior art, the drawbar was secured at one end to the underbody framework of the tractor, usually by means of a pin which engaged a hole formed in a heavy casting of the tractor frame, such as the transmission housing. This pin not only anchored the drawbar but also functioned as the pivot point for the swinging drawbar. The drawbar usually had two additional holes, one close to the fixed end of the drawbar and one spaced a distance therefrom toward the free end of the drawbar; the latter of which would retain the drawbar in its stored position when the pin was inserted therethrough to engage the pivot hole. Adjustment between these two positions required an operator to crawl under the tractor, manually remove the pin, physically reposition the drawbar to the desired position longitudinally, and reinsert the pin into the pivot hole. Such a procedure is cumbersome, inconvenient and time-consuming. Adjustment in the prior art to accommodate different lengths of drive lines on power driven implements almost universally involved the addition of some device to the hitch hole end of the drawbar, as exemplified by U.S. Pat. Nos. 3,557,892, 4,008,905, 4,156,473, and 4,195,704. Such devices are also cumbersome to handle, necessitate storage when not in use and are time consuming to install and remove.
It is, therefore, an object of this invention to provide an adjustable drawbar which is self storing and which permits easy and quick adjustment of its length.
It is also an object of this invention to provide an equal angular adjustable drawbar which permits a full range of adjustments between stored and fully extended to accommodate the longer commercially available drive line without additional parts or material, and which may be adjusted throughout such range without the need for crawling under the tractor.
It is another object of this invention to provide an adjustable drawbar which can be adjusted over a wide range, which is relatively simple and inexpensive to manufacture and install, which is easy and quick to adjust, and which provides visual indication of whether the drawbar is properly adjusted.